Elemental analysis is a method for the determination of carbon, nitrogen, hydrogen, oxygen and/or sulphur composition of different materials, including liquids, solids and gases. During elemental analysis, samples are typically converted to simple gases such as H2, CO, CO2, N2, SO2, and H2O by combustion in a high temperature furnace (usually at or above 1000° C.), usually with aid of catalysts to facilitate the combustion. The combustion products are carried by an inert carrier gas (He or Ar) to a detector. To allow quantitative or qualitative determination of each gas species, the mixture is separated in one or more chromatographic columns, such as a gas chromatographic column or by adsorption/thermodesorption techniques, and detected using for example thermal conductivity detection (TCD), UV fluorescence, optical absorption spectroscopy (UV, Visible or IR), flame photometric detection, atomic absorption spectroscopy, inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), glow discharge mass spectrometry (GD-MS), or by mass spectrometers for isotope ratios.
Typical systems comprise a reactor to convert sample material to simple gases, one or more chemical traps to adsorb undesired gas analytes such as H2O, one or more separation columns and a detector which can for example be a gas sensor and/or a mass spectrometer or one of the other detection systems mentioned above. The flushed volume of reactors and separation devices determines the required carrier gas flow in the system, i.e. the greater the flushed volume the higher is the required gas flow. The carrier gas flow is commonly in the range of 40-300 ml/min, but can be as low as a few mL/min up to 1000 mL/min.
The gaseous combustion products that are to be detected are transported through the system by a carrier gas, such as helium or argon. The carrier gas however dilutes the gas molecules that are generated during sample conversion. As a result, small gas amounts become difficult to detect accurately and precisely; in other words, the signal-to-noise ratio becomes unfavourable.
Concentrating the sample gas prior to detection represents one possible solution to this dilemma. Methods for preconcentrating samples are known in the art. For example, common preconcentrating methods use adsorption and desorption techniques, sometimes in conjunction with cryogenic traps. In general, the adsorption takes place on the surface of an adsorbent. High sample amounts load the traps until the amount of the trapped analyte is sufficient for a significant detector signal. The release of the analyte is commonly controlled by temperature ramps. The desorption time is much shorter than the adsorption time which eventually increases the analyte in the carrier gas flow.
By way of example, Hansen & Sommer (Rapid Commun. Mass Spectrom. 2007; 21: 314-318) describe use of an ashtray system for collecting residual gases, for subsequent detection in a mass spectrometer.
Other preconcentration techniques make use of membranes where the desired analyte gas passes through the membrane while the remaining gas mixture is denied. The carrier gas flow for the analyte can be reduced so to concentrate it. In methods described in U.S. Pat. No. 5,142,143A, the adsorbed gases are released into lower pressure with a low flow of carrier gas where the desorbed gas therefore has a greater density than the carrier gas.
U.S. Pat. No. 6,155,097A describes a system for increasing concentration of trace vapor in a carrier medium, air in this instance by passing it through a membrane gas separator. The gas separator preferentially passes a portion of the trace vapor and rejects all but a very small portion of the carrier medium. The sample, concentrated in trace vapor with respect to the carrier medium, is then compressed by a turbomolecular pump resulting in an increase in density of the trace vapor at the exhaust port of the pump.
Another system known in the art is described in U.S. Pat. No. 6,649,129, which discloses a system for concentrating a gas sample using a cryofocuser, for delivery to a gas chromatograph.
U.S. Pat. No. 4,872,334 discloses an apparatus and method for temperature programmed capillary column gas chromatography. The apparatus is characterized in that it has two flow paths for carrier gas which can join to one flow path before a sample injection device, and one of the flow paths has a valve which can rapidly stop or decrease the flow of the carrier gas in that flow path.
US 2014/0283580 discloses a system for analyzing rare gases, that is based on trapping by means of a getterizing substrate to achieve a superconcentrated rare gase that is subsequently extracted for analysis.
In WO 2011/070574, an apparatus is described that includes a chamber for concentrating at least one analyte in a gaseous sample. Following concentrating the analyte, a carrier gas is used to transfer the concentrated analyte into a chromatographic separator for analysis and subsequent detection.
All of these previously described systems suffer from the drawback of requiring additional devices for concentrating analyte gas, such as traps, membranes or sorbent materials.
The present invention has been made against this background, to provide a system and method for preconcentrating analytes which addresses one or more of the issues mentioned.